Diesel particulates, their effect and control, are at the center of much concern and controversy. Their chemistry and environmental impact present complex issues. Generally, the diesel particulate matter is principally solid particles of carbon and metal compounds with adsorbed hydrocarbons, sulfates and aqueous species. Among the adsorbed species are aldehydes and polycyclic aromatic hydrocarbons (also called PAH's). Some of these organics have been reported to be potential carcinogens or mutagens. Unburned hydrocarbons are related to the characteristic diesel odor and include aldehydes such as formaldehyde and acrolein. The aldehydes, like carbon monoxide, are products of incomplete combustion.
It is not just these organics which are of concern. In one study, diesel particulates were tested along side TiO.sub.2 and carbon without any adsorbed hydrocarbons. (U. Heinrich, et al, "Tierexperimentelle Inhalationsstudien Zur Frage der Tumorinduzierenden Wirkung von Dieselmotorabgasen und zwei Teststauben", Oklolgische Forschung BMFT/GSF, Munich, 1992) The reporters determined that all species tested showed carcinogenic tendency. Until further work clarifies this matter, it would be prudent to look for systems which could control particulates of any composition.
Unfortunately, increasing the recovery of particulates simply by modifying trap design or size would increase the rate of back pressure buildup within the trap. Moreover, control of the various pollutants seems to be interrelated, with reduction of one sometimes increasing levels of another. By modifying combustion to achieve more complete oxidation, decreases can be achieved for pollutants resulting from incomplete combustion, but NO.sub.x is typically increased under these conditions.
NO.sub.x, principally NO and NO.sub.2, contributes to smog, ground level ozone formation and acid rain. NO is produced in large quantities at the high combustion temperatures associated with diesel engines. The NO.sub.2 is formed principally by the post oxidation of NO in the diesel exhaust stream. Several attempts have been made to reduce NO.sub.x, such as by retarding engine timing, exhaust gas recirculation, and the like; however, with current technology, there is a tradeoff between NO.sub.x and particulates. When NO.sub.x is reduced, particulate emissions increase. And, as noted, conditions favoring low emissions of NO.sub.x often favor production of increased levels of CO and HC.
It has been proposed to employ ammonia or an equivalent in a catalytic process to reduce nitrogen oxides in internal combustion engines; however, these systems would not be practical as proposed for diesel exhaust. The presence of high levels of particulates and sulfur would surely adversely affect the catalyst. Therefore, the type of system disclosed by Jones in U.S. Pat. No. 3,599,427 would not be practical for diesel engines.
It is clear that diesel traps (either catalyzed or uncatalyzed) will be required in order to control particulates, especially where efforts are made to control NO.sub.x. However, the use of uncatalyzed traps increases carbon monoxide and the use of catalyzed traps has other disadvantages--notably increases in the discharge of SO.sub.3 and thus total particulates and other problems.
The use of diesel traps and the need to improve them has resulted in a great deal of research and a great number of patents and technical publications. The traps are typically constructed of metal or ceramic and are capable of collecting the particulates from the exhaust and withstanding the heat produced by oxidation of carbonaceous deposits which must be burned off at regular intervals.
This burning off, or regeneration, could occur by itself if the operating temperature of the trap were sufficiently high. However, in the typical situation, the exhaust temperature is not constantly high enough, and secondary measures such as electrically heating to raise the trap temperature or using a catalyst to reduce the combustion temperature of particulates, have not been fully successful.
The use of trap heaters creates an intense load on batteries, especially because they are most needed at lower power settings where the electrical output is also low. The use of catalysts has taken many forms, but none have been found to be fully satisfactory. While catalysts can be very effective in reducing carbon monoxide and unburned hydrocarbons, they can be too easily fouled, have associated health risks, and/or catalyze the oxidation of SO.sub.2 to SO.sub.3 (which then combines with water and increases the weight of particulates), or have two more of these shortcomings.
In a recent assessment of this technology, R. Beckman et al assert that the technical challenge is to find a catalyst which selectively catalyzes the oxidation of carbonaceous components at low exhaust temperatures typical of diesels operating at partial load, and does not oxidize sulfur dioxide or nitrogen oxide at high load temperatures. ("A New Generation of Diesel Oxidation Catalysts", Society of Automotive Engineers (SAE) Paper No. 922330, 1992) They described tests studying the aging of platinum-catalyzed cordierite honeycomb traps, and concluded, inter alia, that the aging was related to adsorption of sulfur and that this depended on both the sulfur content of the fuel and the phosphorous content of the lubricating oil. With control of both of these, aging could be slowed. However, sulfur will remain in diesel fuels, even with planned reduction to 0.05%, and there will remain a need for a means to maintain the activity of catalysts for reducing emissions of carbon monoxide, unburned hydrocarbons, and reducing the ignition temperature of loaded traps.
In "Control of Diesel Engine Exhaust Emissions in Underground Mining", 2nd U.S. Mine Ventilation Symposium, Reno, Nev., Sep. 23-25, 1985, at page 637, S. Snider and J. J. Stekar report that precious metal catalysts in a catalytic trap oxidizer and a "catalyzed Corning trap" were effective in the capture of particulate matter, but both systems increased the conversion of SO.sub.2 to SO.sub.3. The increase in the rate of oxidation of the benign, gaseous dioxide form to the trioxide form results in the adsorption of greater amounts of acid sulfates and associated water onto the particulates discharged. Thus, the weight of the particulates is increased, and the difficulty in reaching regulatory compliance is increased.
The Snider et al report also discussed several other approaches, including the use of a fuel additive containing 80 ppm manganese and 20 ppm copper to reduce the regeneration temperature of the trap. While this was effective in reducing the particulate ignition temperature, these "base metal" catalysts were potentially problematic. Moreover, no measurable reductions in carbon monoxide, unburned hydrocarbons or NO.sub.x were noted.
In "Assessment of Diesel Particulate Control--Direct and Catalytic Oxidation", Society of Automotive Engineers (SAE) Paper No. 81 0112, 1981, Murphy, Hillenbrand, Trayser, and Wasser have reported that the addition of catalyst metal to trapped particulates can decrease particulate ignition temperatures. The catalysts were metal chlorides, including platinum chlorides. The use of halogens in platinum compounds, however, can lead to vaporization of the catalyst metal compound. Moreover, the above Snider et al article indicates that precious metal catalysts could be expected to increase the oxidation of SO.sub.2 to SO.sub.3.
In a 1987 report, R. W. McCabe and R. M. Sinkevitch summarized their studies of diesel traps catalyzed with platinum and lithium, both individually and in combination (Oxidation of Diesel Particulates by Catalyzed Wall-Flow Monolith Filters. 2. Regeneration Characteristics of Platinum, Lithium, and Platinum-Lithium Catalyzed Filters; SAE Technical Paper Series-872137). They noted that carbon monoxide conversion to the dioxide was negligible over the lithium filter, good for platinum, but good only initially for the combined catalyst. They further noted that platinum undergoes a reversible inhibition due to the presence of SO.sub.2, but in the presence of the lithium catalyst there is apparently a wetting of the platinum crystallites by Li.sub.2 O.sub.2. From this work, it can be seen that platinum and lithium on their own help burn out at low temperature, but not necessarily low enough to make supplemental heat unnecessary.
In a more recent report, B. Krutzsch and G. Wenninger discussed their investigation of sodium and lithium-based fuel additives (Effect of Sodium- and Lithium-Based Fuel Additives on the Regeneration Efficiency of Diesel Particulate Filters, SAE Technical Paper Series 922188, 1992). They noted that the predominantly used diesel additives were based on transition metals such as iron, copper, and manganese. They were interested in the sodium and lithium catalysts, however, because of the health concerns with the others. Moreover, the transition metals were seen to form oxides which foul the traps and cannot be easily removed. They found that the sodium and lithium additives did permit regeneration at temperatures low enough to possibly eliminate the need for supplementary heat, and did, therefore, have some promise in improving trap operation as was achieved previously with the transition metal catalysts. However, they also pointed out that there was no effect on the gaseous components, thus both carbon monoxide and unburned hydrocarbon levels remained higher than would be desired.
There is a present need for an improved means for rendering the exhaust from diesel engines more environmentally benign, and, especially to enable this without requiring engine retrofitting, the use of expensive catalytic units, or creating health concerns either from catalysts employed or the production of harmful by-products such as increased levels of sulfates in the discharged particulates.